A while ago I heard Billy Connelly utter an observation that had already troubled me for some time. He noted that we find the smell of farts very offensive, with the exception of our own. This was an anomaly that is well known but not usually explored. We can cheerfully sit immersed in the fug of our own emissions as others flee in desperate attempts to reach oxygen but we reel in horror at even the minor whisperings of their tracts. It's as if our nose can discriminate between 'self' and 'not-self' quite effectively. I usually take the extra step and note that the smell of my own shit is nowhere near as offensive as that of others. To me, it seems to have a relatively innocuous and earthy odour but other members of the family do not agree. At all. In fact, if I find my own solid wastes to smell unpleasant, it indicates that all is not as it should be in my lower reaches. I'm ill. What is the possible origin or advantage of this bizarre trait?
Then a flash of insight came while I was watching our young cat clean himself up. Many animals clean themselves by licking their anuses (anii?), not just because they can but because they have no choice. It's the only way. It removes the possibility of fly strike and reduces their personal odour, important for a hunter. So for them there is a distinct advantage to having no aversion to one's own wastes. Is it possible that some vestige of this strong trait persists through to humans? It would have to have faded with evolution as we have no desire to consume our own, except for a few unfortunate coprophiles but a slight echo of the original gene would explain a great deal.
Of course, dogs don't count. They'll eat any old shit. We have a Border Collie, a female because my wife imagined that a bitch would be better behaved and cleaner of habit. She once excavated and ate some well-fermented donkey manure that I'd just dug into the garden and then came into the house to puke it up all over the rug. Their interest in any shit is usually explained by the fact that a bitch keeps the den clean by eating the pups' droppings, to reduce odours that might attract predators. Frankly I suspect that they are aware of high protein levels in faeces and being natural scavengers, just get it into them. I understand that pigs are equally blasé about their diet.
But that prissy olofactory discrimination between self and other seems to belong to higher orders of being, like humans and cats. Is it possible that this is the answer to one of the great mysteries of human preferences and behaviours? Wouldn't that be nice?